Koi and goldfish can successfully survive winter, but only if their environment remains stable and properly oxygenated. In colder climates, fish do not thrive during winter—they simply endure it. Your role is to minimize stress, maintain water quality, and ensure adequate oxygen exchange until spring.
How winter affects koi and pond fish
- Fish enter a semi-dormant state as water temperatures drop
- Their metabolism slows dramatically
- Their immune system becomes largely inactive
- Fish stop eating once water temperatures fall below 50°F (10°C)
Because of this, even small environmental issues can lead to significant health problems.
Key factors for overwintering success
1. Pond depth and stability
- A minimum depth of 2 ft (24 in / 60 cm) is recommended
- Deeper water remains more stable and slightly warmer at the bottom
- Fish will naturally rest in these deeper zones during winter
2. Oxygen and gas exchange (most critical factor)
Even when ice forms, your pond must still “breathe.”
Without proper gas exchange:
- Oxygen levels drop
- Toxic gases (ammonia, carbon dioxide) build up
Recommended solutions:
-
Aquascape 300 Watt Pond De-Icer
- Keeps a hole open in the ice
- Allows harmful gases to escape
-
Aquascape Aeration Kits
- Provide continuous oxygen and circulation
- Help maintain an opening in the ice
Important: Never break ice with force, as the shock can harm or kill fish.
3. Circulation during winter
- Light circulation or aeration is beneficial
- Avoid excessive disturbance of deeper, warmer water layers
- Position aeration systems higher in the water column, not at the bottom
4. Shut down and protect filtration equipment
- External filters like the UltraKlean™ Pond Filter must be shut down, drained, and stored in freezing conditions to prevent cracking and damage
- Plumbing lines should be completely drained
5. Feeding guidelines
- Stop feeding once water temperatures fall below 50°F (10°C)
- Fish cannot properly digest food in cold water
- Uneaten food will pollute the pond
Use the Aquascape Smart Pond Thermometer to let you know when the temperatures are too low to safely feed your fish!
In fall (before temperatures drop too low), transition to:
-
Aquascape Cold Water Fish Food
- Formulated for easier digestion in cooler temperatures
6. Water quality before winter
Preparing your pond before freeze-up is just as important as winter care.
- Remove leaves and debris
- Perform a partial water change if needed
- Add beneficial bacteria to reduce organic buildup
Recommended:
-
Aquascape Beneficial Bacteria
- Helps break down sludge and organic waste before winter
7. Algae control systems
If using an ion-based system:
-
IonGen™ System G2
- Should be shut down during winter months
- Prevents unnecessary treatment when algae is not actively growing
- Helps extend the life of the probe
8. Common winter myths
- Fish are not frozen in the ice—they remain alive in deeper water
- Parasites and pathogens do not fully stop in winter
- Fish are actually more vulnerable due to weakened immune systems
Summary of best practices
- Maintain at least 2 ft (24 in / 60 cm) of depth
- Keep a hole open in the ice at all times
- Provide aeration or a de-icer
- Stop feeding below 50°F (10°C)
- Remove and store external filtration systems
- Clean and prepare the pond before winter
- Avoid sudden environmental changes
With proper preparation and the right equipment, your koi and pond fish will come through winter safely and be ready for a healthy, active spring.